Diacetyl (2,3-butanedione) and other vicinal diketones and precursors thereof are formed as so-called secondary fermentation products from the yeast during fermentation. Diacetyl is considered to be one of the young bouquet substances which determine the aroma of beer at the unfinished stage. Diacetyl is known in the food industry as synthetic butter aroma. When the flavour threshold value of approximately 0.1 mg/l (0.1 ppm) is exceeded, it provides beer with an impure, sweetish taste (off-flavour) which, when present in relatively high concentration, gives rise to the butter aroma which is usually undesirable in beer.
During fermentation, the yeast used forms pyruvate as one of the main products of the energy metabolism. During this process, acetohydroxy acids are also formed which are released by the yeast to the surrounding medium. Outside of the yeast cells, the vicinal diketones pentanedione and diacetyl are formed from these acetohydroxy acids by oxidative decarboxylation. This process occurs spontaneously without the participation of the yeast.
Diacetyl is an intermediate product in fermentation. The decomposition of diacetyl takes place by yeast which absorbs the diacetyl and uses it as hydrogen acceptor. The diacetyl is reduced by the yeast, via the intermediate stage of acetoin, to form butanediol, a substance which, as a result of its very high flavour threshold value, no longer influences the beer aroma.
The fermentation process for the production of beer usually takes places in two stages: the main fermentation is triggered by the addition of microorganisms, in particular yeasts. During this process, the fermentation substrate, the so-called extract, is essentially subjected to alcoholic fermentation. The main fermentation process usually takes place under warm conditions (at approximately 15 to 22° C.) or with cooling (approximately 5 to 10° C.). The main fermentation process as such is completed when the extract obtained has been completely attenuated. This moment is referred to as the end point of extract fermentation. For the decomposition of the undesirable secondary fermentation products, vicinal diketones, the main fermentation process needs to be maintained after reaching the end point until the total diacetyl content has fallen below the flavour threshold value (threshold of perception). Once the threshold value has been reached, the main fermentation process is terminated, the green beer obtained is cooled down and subjected to a week long secondary fermentation in the cold (at approximately 0 to −1° C.), whereby the beer is clarified. The process of secondary fermentation is also referred to as maturation.
The guide value for the total diacetyl content (vicinal diketones and precursors) for a fully matured beer is as a rule 0.1 mg/l. Consequently, this value is frequently used in breweries as the determining parameter in order to determine the end point of main fermentation. As a rule, this additional period following the end point of extract fermentation is 1 to 4 days.
A disadvantage of known production processes for beer or mixed beer beverages is that the main fermentation process frequently needs to be carried out for longer than is necessary to attenuate the extract. In view of the fact that main fermentation is frequently carried out at a temperature that is elevated (at approximately 15 to 22° C.) or reduced (at approximately 5 to 10° C.) by technical means, the longer main fermentation process is associated with a higher consumption of energy. Moreover, the fermenting tub is blocked for a new fermentation batch for longer. Mostly however, it is precisely the availability of the fermentation tubs which is the limiting factor regarding the capacity of a brewery. It would be desirable to complete the main fermentation process earlier at a point in time at which the extract has been completely or largely attenuated. In order to obtain a beer acceptable from the point of view of flavour, however, the proportion of by-products detrimental to the flavour, such as the total diacetyl content, would have to have fallen to below the threshold of perception. Consequently, there is a requirement for an improved production process for beer or mixed beer beverages in which, above all, the main fermentation process is shortened.